308 Hawg (Jay) might be able to help you out, take a listen to him, he knows a great deal about using the barrel blocks and has helped me out more thaan I can say.
I'm assuming you mean a bbl block and not a bedding block for the action that is, as you mentioned bbl diameter being a consideration...
What diameter is the bbl?
Here's a pic of Jays blocks.
The block sets on four bedded in pillars and is held on by four screws, two at each end. Block is bedded in, action and bbl are free floated.